As you've found, most mechanical ringers will include information on their REN https://en.wikipedia.org/wiki/Ringer_equivalence_number

The ATA should also have some info on it's capability. Most ATA's are capable of driving at least a couple of old style ringers in telephones (REN 1.0 each)

To ring for a specific extension you simply program the ATA to register with that extensions number. Both the extension and the external ringer will ring on an incoming call. If you have the extension set to allow more than one call at a time, the external ringer will still ring if someone is using the extension, something you may, or may not want.